IB 201 LABORATORY SESSION 13 ANIMAL DIVERSITY Goals Introduce students to the diversity of animal Relate aspects of the biology of various animal phyla to their evolutionary relationships Consider the variation within and among animal and use the variation to address the issue of constraints in evolution Assignment 1 Identify the live specimens in the lab to phylum and provide visible characteristics for each 2 Map animal structure onto a phylogeny 3 Write a short assignment that relates the diversity of animal structure and biology to constraints in evolution What does it mean to say that evolution is constrained Introduction Three labs ago we did a survey of the major evolutionary groups of plants This week we will repeat the process for another major group of multi cellular organisms the animals We have no animal equivalent of the greenhouse so we will be using living and preserved specimens in the lab Animals are by far the most diverse group of living organisms in terms of described species with well over one million species that have been named by scientists Animal diversity is very well known relative to most other groups of organisms There are two reasons for this Human beings are animals and it is natural that we have taken more of an interest in the organisms that are most similar to ourselves Also and of more relevance to the study of animal evolution animals have a much greater diversity of body form than any other group of organisms For centuries biologists have been studying the details of development and composition of animal bodies in an attempt to work out the evolutionary relationships among different animal phyla a phylum is a major group of animals such the arthropoda or the mollusca The study of the evolution of animal body form has led to debates about the nature of evolution itself Specifically there has been considerable controversy about the role of animal body plans in constraining evolution Virtually all animal phyla with substantial fossil records first appear very early in the history of animals Some evolutionary biologists have interpreted this pattern as evidence that once basic body plans were established it became very difficult if not impossible to evolve a new body plan from pre existing body plan In other words evolution was constrained so that animals could evolve variations on a theme but not develop an entirely new forms Other biologists have not been convinced by this argument They argue that there is no direct evidence for constraints and that experimental studies of evolution do not support the idea that body plans cannot be changed by the appropriate selective pressure In the following two sections we review some of the terminology used in describing animal structure and an over view of some of the more important animal phyla Students who are also in IB 202 should find some this material familiar although the context in which it is presented is somewhat different Terminology Bilateral vs Radial Symmetry Unlike plants most animal bodies have well defined overall shape Most animals are bilaterally symmetrical the body has a single plane which divides the body into two equal halves Think of the human body It can be divided into equal left and right halves but not equal top and bottom halves or back and front halves Some animals such jellyfish or starfish have radial symmetry in which the body can be divided into equal halves in multiple planes For an analogy think of a pie or a cake they can be cut in half many different ways A few animals such as many sponges are asymmetrical there is no plane that can divide them in two Radial Symmetry Bilaterial Symmetry Coelom The coelom is the body cavity the space in which many of the organs are found A typical animal can be thought of as a hollow tube containing the various organ systems The digestive system then forms a tube within the coelom tube inside a tube body form Some animals lack coeloms they are acoelomate Among animals with coleoms there is variation is how the coelom is formed see below Blastopore During the develop of animal embryos the zygote undergoes mitosis many times to form a ball of cells blastula The blastula then undergoes a process known as gastrulation During gastrulation a pocket forms in the blastula which leads to the formation of a hollow ball of cells with an opening This opening is called a blastopore the ball of cells is called the gastrula The empty space inside the ball of cells becomes the coelom Blastula Gastrulation Gastrula with blastopore Deuterostome An animal in which the blastopore forms the anus and the mouth is formed later Protostome An animal in the blastopore form the mouth and the anus is formed later Pseudocoelom Some animals have a body cavity that forms in a different way Instead of forming from the interior of the gastrula it forms later once different layers of tissue have formed Organ Systems Animals have evolved a series of specialized structures to perform specific functions These structures are called organs Organs are organized into organ systems The major organ systems of animals are nervous system respiratory system circulatory system digestive system excretory system muscular skeletal and reproductive systems Segementation Many animals are segmented Their bodies are divided into a series of distinct sections An earthworm is an obvious and familiar example Lophophore The lophophore is a specialized feeding structure a ring of cilia surrounding the mouth found in a few minor phyla including the Bryozoa The lophophore is used in fllter feeding Animal Ways of Life Habitats Animals live in all major habitats in earth in the ocean fresh water in the soil on land and inside other organisms parasites Feeding Animals feed in a variety of ways Herbivores eat plant material and carnivores eat animal material This can include solid food but also liquid food as well as many animals feed on the body fluids of plants and or animals An important way of life for many marine animals is filter feeding Many animals filter small organisms and pieces of organic material from the water that passes over their bodies Other animals are parasites and live inside of other organisms They generally have greatly reduced digestive systems Reproduction and Development Most animals are capable of sexually reproduction but many are also capable of asexual reproduction e g budding in sea anemones Animals with sexual reproduction can be hermaphrodites individuals are both